A greenhouse structure must provide shelter, access to sunlight, and a favorable atmosphere for growth of plants. On ground level, this generally means constructing a suitable structure of sufficient size, with walls and roof of material allowing transmission of sunlight and limiting loss of heat. A major portion of the total sunlight enters through the roof, particularly in the summer when the sun is more nearly overhead. Major design considerations are minimizing construction and environmental conditioning costs.
However, if it is desired to build a structure to provide a greenhouse environment on the porch or patio of a multi-unit high-rise building, certain other considerations become more important. Structural requirements are more severe in high buildings due to high wind loads, public safety, and building codes. Installation generally must be done entirely from the inside. Because porches are relatively small, the structure must provide maximum use of the available space. It may be desirable to revise the installed structure to enclose more or less of a given porch at a later date, or even to move it to another apartment. Light gathering area is limited. Despite these special requirements, it is still desirable to have the enclosure available to the apartment resident at a minimum cost, which implies the use of standardized mass-produced components in a design adaptable to a variety of porch sizes and configurations.
Porches in most high-rise buildings are built one right above the other, meaning that the floor of one is an opaque roof for the one below. Therefore, the only sunlight which can get inside a porch area must pass through the open perimeter. In winter, the sun does not rise as high above the horizon as it does in the summer. That means that in the winter, the sun shines more directly through the side of the porch than it does in the summer when it is more nearly overhead. As a result, it is during the summer, when a greenhouse should be the most active, that the least sun shines inside a porch.
The actual area of a window (A.sub.o) transmits a maximum area of solar radiation when it is shining perpendicular to the window surface (angle of incidence .theta. equals 0.degree.). As the light moves away from perpendicular to the window surface (angle of incidence .theta. increases), the transmitted area of solar radiation decreases, and can be computed from: EQU A=A.sub.o Cos .theta.
For a vertical south-facing window at latitude 42.degree., the sun reaches a minimum .theta..sub.w =25.degree. at midwinter noon, so that A.sub.w =A.sub.o (0.91). In midsummer, for the same window, .theta..sub.s =71.degree., so that A.sub.s =A.sub.o (0.33). Thus, the window is nearly fully effective in winter, but is effectively only 1/3 of its size, as far as light gathering is concerned, in midsummer.
Light incident on a transparent surface is partially transmitted, partially reflected, and partially absorbed. While materials of low absorptance are readily available, means for coating the glazing to minimize reflectance, though available, are relatively expensive and effective on a limited range of wavelengths. For uncoated glazing, light shining perpendicular to the transparent surface (angle of incidence .theta.=0.degree.), is about 90% transmitted (transmissability T.sub.o =0.9), with little reflected. However, as the light rays are moved more than about 45.degree. away from perpendicular to the transparent surface, increasing the angle of incidence, the amount of light reflected begins to increase. When the angle of incidence .theta. reaches 70.degree., only about 70% of the light is transmitted through the surface (T.sub.70 =0.7). Thus, as the sun goes higher in the sky, the fraction of light reflected from a vertical window increases, leaving a reduced fraction of the light to be transmitted through the window.
Therefore, a vertical window transmits a decreasing amount of light as the sun rises in the sky for two reasons; reduced window area presented to the solar radiation, and increased reflection of the radiation from the surface. For example, for a window at latitude 42.degree., the sunlight admitted, ignoring variations in intensity at earth's surface, varies as follows: EQU I=I.sub.o (Cos .theta.) (T.sub..theta.)
Where:
I=solar radiation passing through the window PA1 I.sub.o =solar radiation (insolation) on a plane normal to the rays PA1 .theta.=angle of incidence PA1 T.sub..theta. =solar transmittance of the glazing material at angle of incidence .theta..
At midwinter noon: EQU I.sub.w =I.sub.o (0.91) (0.90)=(0.82) I.sub.o
And at midsummer noon: EQU I.sub.s =I.sub.o (0.33) (0.70)=(0.23) I.sub.o
Therefore, a vertical south-facing window may transmit only about 1/4 as much sunlight in summer as in winter, which significantly limits its usefulness as a greenhouse glazing.
Structures on the outside of a high-rise building can be exposed to wind loads significantly greater than those at ground level. Structural members and attachment means must therefore be significantly stronger than for those of ground level structures.
While a greenhouse environment can easily be included in the design of a new building, it could also be desirable to add this feature to a porch on an existing building. Porches in a variety of sizes, shapes, and configurations exist on many buildings. Variations include: presence or absence of a roof, presence or absence of one or both side partitions, and open or solid railing structures. The design of a structure for a particular porch must then take into consideration the exact size and configuration of the actual porch, and assemblies must be made to this size in order to provide the weathertight seal required.
A structure being economically added to the side of a high-rise building must satisfy special installation requirements. It must be transported in elements small enough to fit into elevators or stairways, and be capable of being installed, operated, and maintained entirely from the inside.
The ability to grow outdoor plants in any roofed-over porch is severely constrained, even in summer, by the limited amount of sun admitted. Often, only one row of plants is attempted, located along the outer edge of the porch where the sun enters. Due to the high winds and the desire to extend the growing season, a transparent enclosure is desirable.
When it has been desired to incorporate a greenhouse enclosure inside an existing apartment porch, it has been necessary to have an architect, or suitable designer, design a structure to fit the particular porch, generally using components of some construction system which must be determined to be suitable. Then it would be necessary to locate a contractor familiar with both the construction system and the requirements of a greenhouse, to build and install the enclosure. The standard curtain wall system units are not likely to fit inside a typical porch, so that most parts must be custom precut to fit exactly into the required space, or cut at the site, fitted together, and installed.
Special hardware must be located for assembly, windows and, if necessary, doors, all of which are subjected to high wind loads. Materials which are strong and compatible with a greenhouse environment, yet inexpensive, must be found and fitted together. If it is to be heated, materials of high insulating value must be used. In all, considerable planning and design must be done.
In order to capture more sunlight, entire enclosures have been designed and built which are secured to and extended from the surface of the building, so that there is a transparent roof to gather sunlight.